Three-dimensional static random access memory device structures

ABSTRACT

Systems and methods are provided for fabricating a static random access memory (SRAM) cell in a multi-layer semiconductor device structure. An example SRAM device includes a first array of SRAM cells, a second array of SRAM cells, a processing component, and one or more inter-layer connection structures. The first array of SRAM cells are formed in a first device layer of a multi-layer semiconductor device structure. The second array of SRAM cells are formed in a second device layer of the multi-layer semiconductor device structure, the second device layer being formed on the first device layer. The processing component is configured to process one or more input signals and generate one or more access signals. One or more inter-layer connection structures are configured to transmit the one or more access signals to activate the first device layer or the second device layer for allowing access to a target SRAM cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/097,308, filed Dec. 5, 2013, which is incorporated herein byreference in its entirety.

FIELD

The technology described in this disclosure relates generally toelectronic devices and more particularly to memory devices.

BACKGROUND

Static random access memory (SRAM) devices are widely used forelectronic applications where high speed, low power consumption andsimple operations are needed. A SRAM device often includes a number ofmemory cells, and each cell may contain multiple transistors.

SUMMARY

In accordance with the teachings described herein, systems and methodsare provided for fabricating a static random access memory (SRAM) cellin a multi-layer semiconductor device structure. An example SRAM deviceincludes a first array of SRAM cells, a second array of SRAM cells, aprocessing component, and one or more inter-layer connection structures.The first array of SRAM cells are formed in a first device layer of amulti-layer semiconductor device structure. The second array of SRAMcells are formed in a second device layer of the multi-layersemiconductor device structure, the second device layer being formed onthe first device layer. The processing component is configured toprocess one or more input signals and generate one or more accesssignals. One or more inter-layer connection structures are configured totransmit the one or more access signals to activate the first devicelayer or the second device layer for allowing access to a target SRAMcell located in the first array of SRAM cells or in the second array ofSRAM cells.

In one embodiment, a SRAM device includes a first array of SRAM cells, asecond array of SRAM cells, a processing component, and one or moreinter-layer connection structures. The first array of SRAM cells areformed in a first device layer of a multi-layer semiconductor devicestructure. The second array of SRAM cells are formed in a second devicelayer of the multi-layer semiconductor device structure, the seconddevice layer being formed on the first device layer. One or moreinter-layer connection structures are configured to transmit one or moreaccess signals to activate the first device layer or the second devicelayer for allowing access to a target SRAM cell located in the firstarray of SRAM cells or in the second array of SRAM cells. A globalinput/output circuit is configured to output data read from the targetSRAM cell and receive data to be written to the target SRAM cell.

In another embodiment, a method is provided for fabricating a staticrandom access memory (SRAM) device in a multi-layer semiconductor devicestructure. For example, a first array of SRAM cells are formed in afirst device layer of a multi-layer semiconductor device structure. Asecond array of SRAM cells are formed in a second device layer of themulti-layer semiconductor device structure, the second device layerbeing formed on the first device layer. One or more inter-layerconnection structures capable of transmitting one or more access signalsto activate the first device layer or the second device layer forallowing access to a target SRAM cell located in the first array of SRAMcells or in the second array of SRAM cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example diagram of a six-transistor (6-T) SRAM cell.

FIG. 2 depicts an example diagram of a SRAM device.

FIG. 3 depicts an example diagram showing a SRAM device with memorysub-arrays.

FIG. 4 depicts an example diagram showing a SRAM device fabricated in amulti-layer semiconductor device structure.

FIG. 5 depicts another example diagram showing the SRAM device as shownin FIG. 4.

FIG. 6 depicts an example flow chart for fabricating a SRAM device in amulti-layer semiconductor device structure.

DETAILED DESCRIPTION

FIG. 1 depicts an example diagram of a six-transistor (6-T) SRAM cell.As shown in FIG. 1, the SRAM cell 100 includes two pull-up transistors102 and 104 (e.g., P-channel transistors), two pull-down transistors 106and 108 (e.g., N-channel transistors), and two pass-gate transistors 110and 112 (e.g., N-channel transistors). The transistors 102, 104, 106 and108 are connected in cross-coupled inverter configuration and form aflip-flop for storing data. The pass-gate transistors 110 and 112 areboth coupled to a word line 122 that corresponds to a row in a memoryarray including the SRAM cell 100. Two complementary bit lines 124 and126 are coupled to the pass-gate transistors 110 and 112 respectively.

Usually, the SRAM cell 100 operates in a read mode, a write-enable mode,or a power-down mode (i.e., a data-retention mode). In a read mode, thepass-gate transistors 110 and 112 are turned on in response to aword-line signal from the word line 122 to perform a read operation. Adata bit stored in the SRAM cell 100 may be read out through the bitlines 124 and 126. In a write-enable mode, a write operation isperformed to write a new data bit to the SRAM cell 100 through the bitlines 124 and 126 when a word-line signal from the word line 122 turnson the pass-gate transistors 110 and 112. Furthermore, in the power-downmode, the pass-gate transistors 110 and 112 are turned off, and the databit is stored in the SRAM cell 100.

FIG. 2 depicts an example diagram of a SRAM device. As shown in FIG. 2,the SRAM device 200 includes multiple SRAM banks 202 ₁, 202 ₂, . . . ,202 _(N), where N is an integer larger than 1. For example, within theSRAM bank 202 ₁, a SRAM array 204 contains multiple SRAM cells 206disposed at intersections of rows and columns of the SRAM array 204. Aplurality of local bit lines 208 are arranged along the columns of theSRAM array 204, and a plurality of word lines (not shown in FIG. 2) arearranged along the rows of the SRAM array 204. Through certain selectioncircuitry, a column multiplexer 210 selects a subset of the local bitlines 208 for access. One or more global bit lines 212 are coupled tothe local bit lines 208 through the column multiplexer 210 andwrite-driver/sense-amps circuits 214. An input/output data buffer 216provides buffers for reading data out from and/or receiving input datato the SRAM device 200. For example, as shown in FIG. 2, the global bitlines 212 are also used by the SRAM banks 202 ₂, . . . , 202 _(N). As anexample, the SRAM cells 206 include six-transistor SRAM cells (e.g., theSRAM cell 100), eight-transistor SRAM cells, or other types of SRAMcells.

As shown in FIG. 2, the lengths of the local bit lines 208 and thelengths of the global bit lines 212 both affect the speed of the SRAMdevice 200. One approach to increase the speed of a SRAM device is toshorten the local bit lines 208.

FIG. 3 depicts an example diagram showing a SRAM device with memorysub-arrays. As shown in FIG. 3, in a SRAM device 300, a SRAM array isdivided into multiple sub-arrays (e.g., SRAM sub-arrays 302 and 304).For example, each sub-array has a 4×144 configuration, i.e., 4 rows and144 columns. Each row corresponds to a word line, and each columncorresponds to one or more local bit lines. As an example, within thesub-array 302, one or more word lines 306 are arranged in a horizontaldirection, and one or more local bit lines 308 are arranged in avertical direction.

Global control (MCTL) circuits (e.g., the MCTL circuit 324) areconfigured to perform certain control functions, such as pre-decodinginput signals (e.g., address signals, read/write signals, and/or clocksignals) for accessing a target memory cell. Global input/output (GIO)circuits (e.g., the GIO circuit 322) are configured to read out datastored in a memory cell or receive data to be written to a memory cell.Each sub-array in the SRAM device 300 is associated with a localinput/output (LIO) circuit, a local control (LCTL) circuit, and aword-line driver (WLD4). For example, the sub-array 302 is associatedwith a LIO circuit 310, a LCTL circuit 312 and a WLD4 320. The LIOcircuit 310 and the WLD4 320 are configured to access one or more memorycells in the sub-array 302, e.g., based on the pre-decoded inputsignals. The WLD4 320 is configured to activate a particular word line,and the LIO circuit 310 is configured to activate one or more particularlocal bit lines so that one or more memory cells corresponding to theactivated word line and the activated bit lines can be accessed toperform read/write operations. The LIO circuit 310 is further configuredto communicate with the GIO circuit 322 through a global bit line (GBL)314. The LCTL circuit 312 is configured to communicate with the MCTLcircuit 324 through a global word-line-decoder (WDECC) line 316. Forexample, the LCTL circuit 312 receives the pre-decoded input signalsfrom the MCTL circuit 324, determines information associated with aparticular word line and/or one or more particular local bit lines to beactivated, and provides the information to the LIO circuit 310 and theWLD4 320.

As shown in FIG. 3, dividing the SRAM array into multiple smallsub-arrays often requires, for each sub-array, a LIO circuit and a LCTLcircuit which occupy more areas on the chip. Furthermore, thoughdividing the SRAM array into multiple small sub-arrays results inshortened local bit lines, the global bit lines (GBL) and the globalword-line-decoder (WDECC) lines are prolonged. For example, the globalbit line 314 as shown in FIG. 3 needs to extend over not only threesub-arrays but also two LIO circuits, and the WDECC line 316 needs toextend over not only four sub-arrays but also two local controlcircuits. In contrast, neither the LIO circuits (e.g., the LIO circuit310) nor the local control circuits (e.g., the local control circuit312) are needed for an undivided SRAM array, and the undivided SRAMarray would have a global bit line extending over three sub-arrays and aWDECC line extending over four sub-arrays. The benefits of shortenedlocal bit lines in the SRAM device 300 with respect to increasing speedare reduced due to the prolonged global bit lines and the prolongedWDECC lines.

FIG. 4 depicts an example diagram showing a SRAM device fabricated in amulti-layer semiconductor device structure. As shown in FIG. 4, the SRAMdevice 400 includes multiple device layers (e.g., the layers 402 ₁-402_(n), where n is an integer larger than 1). Each device layer includes anumber of SRAM cells arranged in columns and rows, a word-line (WL)driver, a LIO circuit, and a layer decoder. For example, the devicelayer 402 ₁ includes a word-line (WL) driver 404, a LIO circuit 406, anda layer decoder 408. In addition, the device layer 402 includes one ormore word lines 410 arranged along a first direction, and one or morelocal bit lines 412 arranged in a second direction that is perpendicularto the first direction.

One or more global bit lines (GBLs) 414 extend across all differentdevice layers (e.g., the layers 402 ₁-402 _(n)) and connect with the LIOcircuits (e.g., the LIO circuit 406) in the different device layers. Inaddition, one or more word-line-decoder (WDECC) lines 416 extend acrossall different device layers (e.g., the layers 402 ₁-402 _(n)) andconnect with the WL drivers (e.g., the WL driver 404) in the differentdevice layers. Furthermore, the SRAM device 400 includes a globalcontrol (CNT) circuit 420 configured to perform certain controlfunctions and a global input/output (GIO) circuit 418 configured to readout data stored in a memory cell or receive data to be written to amemory cell.

In some embodiments, a particular device layer (e.g., one of the layers402 ₁-402 _(n)) includes two groups of SRAM cells forming a butterflystructure. Specifically, the LIO circuit and the layer decoder of theparticular device layer are located between the two groups of SRAM cellseach arranged in columns and rows. For example, the two groups of SRAMcells have a same number of rows and a same number of columns. Incertain embodiments, each of the device layers (e.g., the layers 402₁-402 _(n)) includes two groups of SRAM cells forming a butterflystructure. In some embodiments, the CNT circuit 420 and/or the GIOcircuit 418 are located on top of or underneath the SRAM cells in aparticular device layer (e.g., one of the layers 402 ₁-402 _(n)). Incertain embodiments, the LIO circuit and/or the layer decoder of aparticular device layer (e.g., any one of the layers 402 ₁-402 _(n)) arelocated on top of or underneath the SRAM cells in the same device layer.

FIG. 5 depicts another example diagram showing the SRAM device 400. Asshown in FIG. 5, the GIO circuit 418 includes a write-in component 502configured to receive data to be written to a target SRAM cell, aread-out component 504 configured to receive data read from a targetSRAM cell, and an in/out latch 506 configured to temporarily store inputor output data. The CNT circuit 420 includes a pre-decoder 508configured to pre-decode certain input signals, such as address signals,read/write signals, and clock signals. After the input signals arepre-decoded, information associated with a target SRAM cell to beaccessed for read/write operations is obtained, such as the device layeron which the target memory cell is located, and the row and the columncorresponding to the target memory cell.

For example, based on the information obtained through pre-decoding theinput signals, it is determined that the target SRAM cell is located onthe device layer 402 ₁. In response, the layer decoder 408 is used toactivate the device layer 402 ₁. Then, the WL driver 404 and the LIOcircuit 406 are configured to activate a particular word line and one ormore particular local bit lines for accessing the target SRAM cell. Asshown in FIG. 5, the LIO circuit 406 includes a column multiplexer 510,a local-read-out/local-sense-amplifier component 512, and a local writebuffer 514. The column multiplexer 510 is configured to select aparticular column associated with the target SRAM cell. Thelocal-read-out/local-sense-amplifier component 512 is configured tostore data read from the target SRAM cell. In addition, the local writebuffer 514 is configured to store data to be written to the target SRAMcell.

Referring to FIG. 4 and FIG. 5, through the GBL 414, the LIO circuit 406sends data read from the target SRAM cell to the GIO circuit 418, orreceives data to be written to the target SRAM cell from the GIO circuit418. In some embodiments, data to be written to the target SRAM cell istransmitted to the LIO circuit 406 using an inter-layer structure otherthan the GBL 414. The layer decoder 408 communicates with the CNTcircuit 420 through the WDECC line 416.

As an example, the SRAM cells in the SRAM device 400 includesix-transistor SRAM cells (e.g., the SRAM cell 100), eight-transistorSRAM cells, or other types of SRAM cells. For example, more metalporosity is achieved since the global bit lines made of certain metalmaterials penetrate different layers in the SRAM device 400. In someembodiments, the GIO circuit 418 and the CNT circuit 420 are placedtogether in one of the device layers 402 ₁-402 _(n). In certainembodiments, the GIO circuit 418 and the CNT circuit 420 are placedtogether in a separate device layer other than the device layers 402₁-402 _(n). In other embodiments, the GIO circuit 418 and the CNTcircuit 420 are placed in different device layers.

In some embodiments, the SRAM device 400 includes two device layers(e.g., n=2) and each device layer includes a SRAM array. The length ofthe GBL 414 and the length of the WDECC line 416 are comparable with asum of the heights of the two device layers. In some embodiments, theglobal bit line 414 and the WDECC line 416 are much shorter than theglobal bit line 314 and the WDECC line 316 as shown in FIG. 3,respectively. For example, in each layer, an 1152-cell pitch is usedalong the first direction (i.e., the direction of the word line 410),and a 4-cell pitch is used along the second direction (i.e., thedirection of the bit lines 412). The length of the word line 410 isapproximately equal to the length of the 1152-cell pitch.

In certain embodiments, the SRAM device 400 includes eight device layers(e.g., n=8). The length of the global bit line 414 and the length of theWDECC line 416 are comparable with a sum of the heights of the eightdevice layers. In certain embodiments, the global bit line 414 and theWDECC line 416 are shorter than the global bit line 314 and the WLDVdecoder 316 as shown in FIG. 3, respectively. For example, in eachlayer, a 288-cell pitch is used along the first direction (i.e., thedirection of the word line 410), and a 4-cell pitch is used along thesecond direction (i.e., the direction of the bit line 412). Then, thelength of the word line 410 is approximately equal to the length of the288-cell pitch.

In certain embodiments, the GIO circuit 418 is located between any twoof the device layers (e.g., the layers 402 ₁-402 _(n)). Specifically,the GIO circuit 418 is located between the LIO circuits of the twodevice layers, as an example. In some embodiments, the CNT circuit 420is located between any two of the device layers (e.g., the layers 402₁-402 _(n)). Specifically, the CNT circuit 420 is located between thelayer decoders of the two device layers, as an example.

FIG. 6 depicts an example flow chart for fabricating a SRAM device in amulti-layer semiconductor device structure. At 602, a first array ofSRAM cells are formed in a first device layer of a multi-layersemiconductor device structure. At 604, a second array of SRAM cells areformed in a second device layer of the multi-layer semiconductor devicestructure, the second device layer being formed on the first devicelayer. For example, the first array of SRAM cells and the second arrayof SRAM cells are arranged in columns and rows in the first device layerand the second device layer respectively. In each of the first devicelayer and the second device layer, one or more word lines are arrangedalong a first direction, and one or more local bit lines are arranged ina second direction that is perpendicular to the first direction.

At 606, one or more inter-layer connection structures are formed totransmit one or more access signals to activate the first device layeror the second device layer for allowing access to a target SRAM celllocated in the first array of SRAM cells or in the second array of SRAMcells. For example, after certain input signals are pre-decoded,information associated with a target SRAM cell to be accessed forread/write operations is obtained, such as the device layer on which thetarget memory cell is located, and the row and the column correspondingto the target memory cell. If it is determined that the target SRAM cellis located on the first device layer, the first device layer isactivated, and then a particular word line and one or more particularlocal bit lines in the first device layer are activated for accessingthe target SRAM cell.

This written description uses examples to disclose embodiments of thedisclosure, include the best mode, and also to enable a person ofordinary skill in the art to make and use various embodiments of thedisclosure. The patentable scope of the disclosure may include otherexamples that occur to those of ordinary skill in the art. One ofordinary skill in the relevant art will recognize that the variousembodiments may be practiced without one or more of the specificdetails, or with other replacement and/or additional methods, materials,or components. Further, persons of ordinary skill in the art willrecognize various equivalent combinations and substitutions for variouscomponents shown in the figures. For example, certain transistors aredescribed herein as examples, and the concepts, structures, layouts,materials, or operations may also be applicable to other types ofsemiconductor devices, such as bipolar junction transistors, diodes,capacitors, etc.

Well-known structures, materials, or operations may not be shown ordescribed in detail to avoid obscuring aspects of various embodiments ofthe disclosure. Various embodiments shown in the figures areillustrative example representations and are not necessarily drawn toscale. Particular features, structures, materials, or characteristicsmay be combined in any suitable manner in one or more embodiments. Thepresent disclosure may repeat reference numerals and/or letters in thevarious examples, and this repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed. Various additionallayers and/or structures may be included and/or described features maybe omitted in other embodiments. Various operations may be described asmultiple discrete operations in turn, in a manner that is most helpfulin understanding the disclosure. However, the order of descriptionshould not be construed as to imply that these operations arenecessarily order dependent. In particular, these operations need not beperformed in the order of presentation. Operations described herein maybe performed in a different order, in series or in parallel, than thedescribed embodiments. Various additional operations may be performedand/or described. Operations may be omitted in additional embodiments.

This written description and the following claims may include terms,such as “on,” that are used for descriptive purposes only and are not tobe construed as limiting. The embodiments of a device or articledescribed herein can be manufactured, used, or shipped in a number ofpositions and orientations. For example, the term “on” as used herein(including in the claims) may not necessarily indicate that a firstlayer/structure “on” a second layer/structure is directly on and inimmediate contact with the second layer/structure unless such isspecifically stated; there may be one or more third layers/structuresbetween the first layer/structure and the second layer/structure. Theterm “semiconductor device structure” used herein (including in theclaims) may refer to shallow trench isolation features, poly-silicongates, lightly doped drain regions, doped wells, contacts, vias, metallines, or other types of circuit patterns or features to be formed on asemiconductor substrate.

What is claimed is:
 1. A static random access memory (SRAM) devicecomprising: a first array of SRAM cells formed in a first device layerof a multi-layer semiconductor device structure, the first device layerfurther comprising a first word-line driver and a first localinput/output circuit coupled to the first array of SRAM cells, the firstdevice layer further comprising a first layer decoder coupled to thefirst word-line driver and the first local input/output circuit; asecond array of SRAM cells formed in a second device layer of themulti-layer semiconductor device structure, the second device layerfurther comprising a second word-line driver and a second localinput/output circuit coupled to the second array of SRAM cells, thesecond device layer further comprising a second layer decoder coupled tothe second word-line driver and the second local input/output circuit,the second device layer being formed on the first device layer; and oneor more inter-layer connection structures configured to transmit one ormore access signals to activate the first device layer or the seconddevice layer for allowing access to a target SRAM cell located in thefirst array of SRAM cells or in the second array of SRAM cells; whereinthe first and second layer decoder are each configured to activate therespective word-line driver and the respective local input/outputcircuit of the respective device layer in response to the one or moreaccess signals indicating that the target SRAM cell is located in therespective device layer.
 2. The SRAM device of claim 1, wherein: thefirst device layer includes one or more first local access circuits; andthe second device layer includes one or more second local accesscircuits.
 3. The SRAM device of claim 2, wherein: the one or more firstlocal access circuits include the first layer decoder, the first localinput/output circuit, and the first word-line driver; the first localinput/output circuit is configured to, in response to the target SRAMcell being located in the first device layer, activate one or more firstlocal bit lines associated with the target SRAM cell; and the firstword-line driver is configured to, in response to the target SRAM cellbeing located in the first device layer, activate a first word lineassociated with the target SRAM cell.
 4. The SRAM device of claim 2,wherein: the one or more second local access circuits include the secondlayer decoder, the second local input/output circuit, and the secondword-line driver; the second local input/output circuit is configuredto, in response to the target SRAM cell being located in the seconddevice layer, activate one or more second local bit lines associatedwith the target SRAM cell; and the second word-line driver is configuredto, in response to the target SRAM cell being located in the seconddevice layer, activate a second word line associated with the targetSRAM cell.
 5. The SRAM device of claim 1, further comprising: a thirdarray of SRAM cells formed in a third device layer of the multi-layersemiconductor device structure, the third device layer being formed onthe second device layer; wherein the one or more inter-layer connectionstructures are further configured to transmit the one or more accesssignals to activate the third device layer in response to the targetSRAM cell being located in the third device layer for allowing access tothe target SRAM cell.
 6. The SRAM device of claim 1, further comprising:a global input/output circuit configured to output data read from therespective local input/output circuit of the target SRAM cell andreceive data to be written to the respective local input/output circuitof the target SRAM cell.
 7. The SRAM device of claim 6, wherein the oneor more inter-layer connection structures are further configured totransmit data read from the respective local input/output circuit of thetarget SRAM cell to the global input/output circuit and transmit data tobe written from the global input/output circuit to the respective localinput/output circuit of the target SRAM cell.
 8. The SRAM device ofclaim 6, wherein the global input/output circuit is formed in a thirddevice layer of the multi-layer semiconductor device structure.
 9. TheSRAM device of claim 1, wherein the one or more inter-layer connectionstructures are formed in a third device layer of the multi-layersemiconductor device structure.
 10. The SRAM device of claim 1, whereinthe one or more inter-layer connection structures include one or moreglobal bit lines and one or more word-line-decoder lines, the global bitlines and the word-line-decoder lines being configured to communicatewith one or more first local access circuits in the first device layerand one or more second local access circuits in the second device layer.11. The SRAM device of claim 1, wherein the one or more inter-layerconnection structures extend across the first device layer and thesecond device layer.
 12. The SRAM device of claim 1, wherein: the firstarray of SRAM cells are arranged in a number of first rows and a numberof first columns, the first rows being associated with a number of firstword lines, the first columns being associated with a number of firstlocal bit lines; and the second array of SRAM cells are arranged in anumber of second rows and a number of second columns, the second rowsbeing associated with a number of second word lines, the second columnsbeing associated with a number of second local bit lines.
 13. The SRAMdevice of claim 1, wherein the first and second word-line driver areeach configured to assert a word line associated with the target SRAMcell in response to activation of the respective word-line driver. 14.The SRAM device of claim 1, wherein the one or more inter-layerconnection structures comprises: a controller configured to receive aninput signal indicating an address of the target SRAM cell, to decodethe address of the target SRAM cell to produce the one or more accesssignals, and to transmit the one or more access signals to the first andsecond layer decoder.
 15. A static random access memory (SRAM) devicecomprising: a first array of SRAM cells formed in a first device layerof a multi-layer semiconductor device structure, the first device layerfurther comprising a first word-line driver and a first localinput/output circuit coupled to the first array of SRAM cells, the firstdevice layer further comprising a first layer decoder coupled to thefirst word-line driver and the first local input/output circuit; asecond array of SRAM cells formed in a second device layer of themulti-layer semiconductor device structure, the second device layerfurther comprising a second word-line driver and a second localinput/output circuit coupled to the second array of SRAM cells, thesecond device layer further comprising a second layer decoder coupled tothe second word-line driver and the second local input/output circuit,the second device layer being formed on the first device layer; one ormore inter-layer connection structures configured to transmit one ormore access signals to activate the first device layer or the seconddevice layer for allowing access to a target SRAM cell located in thefirst array of SRAM cells or in the second array of SRAM cells; and aninput/output circuit configured to output data read from the target SRAMcell and receive data to be written to the target SRAM cell; wherein thefirst and second layer decoder are each configured to activate therespective word-line driver and the respective local input/outputcircuit of the respective device layer in response to the one or moreaccess signals indicating that the target SRAM cell is located in therespective device layer.
 16. The SRAM device of claim 15, furthercomprising: a third array of SRAM cells formed in a third device layerof the multi-layer semiconductor device structure, the third devicelayer being formed on the second device layer; wherein the one or moreinter-layer connection structures are further configured to transmit theone or more access signals to activate the third device layer inresponse to the target SRAM cell being located in the third device layerfor allowing access to the target SRAM cell.
 17. The SRAM device ofclaim 15, wherein the input/output circuit is a global input/outputcircuit and is formed in a third device layer of the multi-layersemiconductor device structure.
 18. The SRAM device of claim 15,wherein: the first device layer includes one or more first local accesscircuits; and the second device layer includes one or more second localaccess circuits.
 19. The SRAM device of claim 18, wherein: the one ormore first local access circuits include the first layer decoder; theone or more second local access circuits include the second layerdecoder; the first layer decoder is configured to activate the firstdevice layer in response to the one or more access signals indicatingthat the target SRAM cell is located in the first device layer; and thesecond layer decoder is configured to activate the second device layerin response to the one or more access signals indicating that the targetSRAM cell is located in the second device layer.
 20. A method forfabricating a static random access memory (SRAM) device comprising:forming a first array of SRAM cells in a first device layer of amulti-layer semiconductor device structure, the first device layerfurther comprising a first word-line driver and a first localinput/output circuit coupled to the first array of SRAM cells, the firstdevice layer further comprising a first layer decoder coupled to thefirst word-line driver and the first local input/output circuit; forminga second array of SRAM cells in a second device layer of the multi-layersemiconductor device structure, the second device layer furthercomprising a second word-line driver and a second local input/outputcircuit coupled to the second array of SRAM cells, the second devicelayer further comprising a second layer decoder coupled to the secondword-line driver and the second local input/output circuit, the seconddevice layer being formed on the first device layer; and forming one ormore inter-layer connection structures capable of transmitting one ormore access signals to activate the first device layer or the seconddevice layer for allowing access to a target SRAM cell located in thefirst array of SRAM cells or in the second array of SRAM cells; whereinthe first and second layer decoder are each configured to activate therespective word-line driver and the respective local input/outputcircuit of the respective device layer in response to the one or moreaccess signals indicating that the target SRAM cell is located in therespective device layer.